The pH influenced the viscosity of a model system of tomato product which consisted of citrus pectin and bovine serum albumin. The viscosity change was completely reversible depending on the pH. The same pH effect was observed in tomato products. Since the amino acid compositions of proteins suggest that the electrostatic property of water-soluble protein fraction from tomato juice can be similar to that of bovine serum albumin, the comparison of pH effects indicates that pectin-protein interaction may be involved in consistency of tomato products. Denaturation of protein may result in a decrease in the serum viscosity during processing.